The present invention relates to ultrasound and, more particularly, to a system and method providing for improved coupling ultrasound between a transducer and a window of an ultrasound probe.
Ultrasonic imaging is widely used to analyze the internal structure of organisms. For example, ultrasound is often employed to characterize the status of a fetus in a pregnant woman. Ultrasonic imaging is based on the detection of reflections of ultrasonic waves at boundaries characterized by unequal impedances. Such boundaries can represent bones, organ boundaries, changes in tissue type, etc.
Typically, ultrasonic imaging is performed using an ultrasonic probe electrically coupled to an electronics module. The probe generally comprises a body serving as a handle, a cap or window which can pressed against the skin of a subject being imaged, and a electro-acoustic transducer enclosed by the body and window. The electronics module generates electrical pulses to be converted to ultrasonic pulses that are propagated through the window and into the subject.
A single ultrasonic pulse can result in multiple reflections due to multiple impedance boundaries along its path of propagation. As these reflections are detected by the probe, they are converted by the transducer to an electrical signal which represents depth by time and impedance mismatches by amplitude. The electronics module analyzes this signal to recover the imaging information which can then be displayed and/or recorded as desired.
The quality of the image obtained is largely dependent on the sensitivity with which the probe can detect reflections. A substantial portion of the energy of an ultrasonic pulse is absorbed by the probe or the body. The remaining energy is distributed among multiple reflections. Only a small fraction of each reflection is directed toward the probe, and much of that small fraction is absorbed before reaching the transducer. The transducer must be able to detect the occurrences and amplitude of these reflections, despite the small amounts of energy in each reflection.
Sensitivity is a function of the aperture, or energy-gathering area, of the transducer. A transducer with a large aperture can receive a greater portion of reflected acoustic energy. On the other hand, a larger aperture implies a shallower depth of focus. A transducer is shaped and/or operated so that there is, at any given time, a single depth at which the transducer's ability to resolve depth is at a maximum. In practice, maximal resolution is not necessary, but some threshold resolution below this maximum can be required by many imaging applications. When a transducer with a small aperture is used, the range of depths for which a given threshold is met or exceeded is larger than the corresponding range of depths available when a large aperture is used.
When the range of depths of interest is greater than the depth of field of a probe, it is necessary to obtain imaging information using focal points at successive depths. Finer steps between focal points are required for a larger aperture. Herein, the process of changing the focal length of a probe during image gathering is referred to as "zooming".
Zooming permits high resolution imaging along a single trajectory. To obtain a two-dimensional image of a "slice" of a subject, the direction of ultrasound propagation must be panned, i.e., swept transversely or "steered". It is this steering action that gives many ultrasound images their fan-shaped form. Herein, steering and zooming are collectively referred to as "scanning".
Scanning is performed differently by various probe types. A small aperture probe with a spherical transducer can rely on a fixed focus and mechanical steering for imaging. Theoretically, a single element transducer could be mechanically deformed to provide for zooming and, thus, larger apertures. However, annular array transducers have been developed in which time delays between concentric elements provide the zooming function; annular array transducers generally employ mechanical steering. Just as phased-arrays are used in radar, it is possible to implement a rectangular array ultrasound transducer in which all scanning is performed electronically. Such rectangular arrays involve considerable processing complexity and are not widely used. Linear phased arrays are simpler to implement and also permit electronic zooming and steering; however, elevational resolution (transverse to depth and pan) is poor.
While each probe design has its advantages, the annular array stands out for allowing high resolution imaging in all directions while demanding less in the way of processing to generate an image from the received reflections. Zooming can be performed electronically at very high speeds for each mechanically controlled pan position.
One challenge in designing large aperture, mechanically scanned ultrasonic transducers such as an annular array transducers is to couple ultrasound transmissions between the probe and the subject optimally. For obvious reasons, including subject comfort, the moving transducer cannot be in intimate contact with the subject. Instead, intimate contact with the subject is made by the probe window. The window material is selected to be safe, comfortable, rigid and transmissive of ultrasonic energy. A more subtle criterion is the requirement that the acoustic refractive index at ultrasound frequencies be closely matched to the subject being imaged. In other words, the acoustic velocities of window and subject should be matched. The purpose of this matching is to minimize image distortion due to changes in beam direction at the subject-window boundary.
In addition, it is desirable to match the ultrasonic impedance of the window to the subject to minimize reflections at that surface. Such reflections bear no useful information, create reflections internal to the probe which can interfere with image clarity, and dissipate energy which could otherwise contribute to useful reflections. However, some compromise in impedance matching is tolerated to accommodate other criteria, particularly rigidity of the window.
A fluid medium is typically interposed between a mechanically panned transducer and the associated probe window to permit steering motion while providing appropriate ultrasonic coupling between the transducer and the window and subject. The requirements for coupling include matching of transmission velocity and impedance among the fluid, window and subject for the same reasons discussed above with respect to matching the window to the body. In addition, the attenuation of the fluid must be considered to balance the requirement of efficient transmission of ultrasound and the need to damp reflections internal to the probe which could create image artifacts and otherwise degrade image quality.
These requirements generally constrain selection of the medium material to be a liquid sealed in a chamber defined by the probe body and window. It is difficult to determine the range of coupling fluids used in mechanically scanned probes, since many of these are proprietary. Various organic liquids have been tried. Frequently, materials are mixed in an attempt to combine the characteristics of each component. However, such most combinations interact in a non-linear manner, rendering the outcome of a mixture unpredictable. While tables of attenuation and impedance are available, selection of a coupling fluid is generally a matter of trial and error.
One problem with known coupling fluids is that it is difficult to vary one parameter of interest, e.g., impedance, without affecting another, e.g., velocity. Often, it is not possible to "tweak" a fluid mixture to obtain the desired properties. Moreover, these properties must be maintained within acceptable tolerances over a range of operating tempertures, further excluding otherwise acceptable coupling fluids.
A number of different materials, e.g., silicone-based oils or mixtures of Glycerol with Propylene Glycol, have been successfully employed as coupling fluids for small aperture probes. However, images produced by larger aperture probes using the same fluids have been plagued by artifacts apparently due to internal reflections. What is needed is an ultrasound system and method for producing clearer images when using large aperture probes. Preferably, such a system and method would employ a coupling fluid for which one parameter of interest can be adjusted without significantly changing another parameter.